1. Field of the Invention
This invention relates to a CMOS multiplexer circuit capable of receiving a plurality of digital signals and then producing an analog potential corresponding to those digital signals.
2. Description of the Related Art
To drive the liquid-crystal panels of image processing systems, for example, CMOS multiplexer circuits are generally used to supply a suitable driving potential to the panels. The multiplexer circuit is supplied with analog potentials of different magnitudes, and is also supplied with a select signal, mostly a digital signal. The select signal selects and turns on one of the supply gates each connected to the analog potentials to supply an analog potential to, for example, the liquid-crystal panel connected.
when the digital select signals pass through the logic circuits for decoding, the transfer gates are not always controlled by the respective control signals with the same timing. Delays in timing are ascribed to the circuit configuration of the inverter circuits or NAND circuits for logical processing, the capacitance, and others. Such timing delay leads to the faulty operation of the multiplexer's supply gates turning on at the same time. In the multiplexer circuit, when the supply gates turn on simultaneously, this causes the analog potentials to short-circuit one another, resulting in the improper output. The change of the well potential due to the short circuit makes the potential of the p-type substrate higher than the n-type substrate potential, creating the problem of permitting the CMOS IC of the multiplexer to latch up.